Implantable blood pumps or artificial hearts have been the subject of significant work for several decades. Although much progress has been made, no approach has demonstrated the high reliability needed for an actuator. Generally, most existing approaches employ conversion of the motion of a rotary electric motor into the linear motion of a pusher plate to squeeze blood from rubber-type ventricles. Some employ a hydraulic piston to squeeze the ventricles with fluid, while some push on the ventricles directly using no hydraulics. Such rotary-to-linear conversion mechanisms, including lead screws, gear pumps and a host of other designs, are all prone to primary component wear and breakdown. Thus, most, if not all, existing actuator approaches have undesirable reliability concerns associated therewith. In fact, existing actuator approaches comprise a major stumbling block in attaining a highly reliable, light weight prosthesis.
One type of artificial heart is depicted in FIGS. 1, 2a & 2b. This total artificial heart (TAH) 10, provided by the Cleveland Clinic, includes blood inflow ports and valves 12 and blood outflow ports and valves 14. As best shown in FIGS. 2a & 2b, the TAH 10 includes two blood pumps, a right blood pump or ventricle 20 and a left blood pump or ventricle 22 within a housing 18. Each blood pump 20 & 22 includes a reciprocable diaphragm 21 & 23, respectively, which is mechanically coupled to a corresponding pusher plate 21' & 23' powered by an interventricular energy converter or actuator 26. Actuator 26, which pilots a guide pin 25 affixed to pusher plate 21' and a guide pin 27 affixed to pusher plate 23', produces an axial reciprocating motion which, during eject mode, drives one pusher plate 21' or 23' towards the TAH 10 housing 18.
The heart is controlled by responding to venous pressure, because more flow is required as pressure increases. The follower is not directly coupled to either pusher plate. Thus, while blood from one blood pump is being ejected, the other is free to fill, with the rate of filling depending on venous return pressure. During fill, each guide pin is free to slide within the actuator, so diaphragm fill cycle motion is determined by venous pressure, rather than the actuator rate. Control logic senses the velocity or position of the diaphragm, and maintains an actuator speed sufficient to avoid fill cycle contact between pusher plate and actuator, without running so fast that efficiency or operation of the opposite pump is impacted. A ventricular assist device operates similarly, except that only one pump is involved.
The TAH 10 can be equivalently operated by a number of different, existing interventricular actuators. For example, most existing electromechanical actuators could be employed. The Cleveland Clinic--type TAH conventionally employs an electrohydraulic energy conversion apparatus. This apparatus comprises a brushless DC motor which turns a gear pump that provides hydraulic flow at about 100 psi. Internal valving controls flow to a double-ended hydraulic actuator. To ensure that the system is hermetically sealed, the actuator piston is actually a stack of magnets riding in the cylinder, with a follower magnet outside the cylinder to match piston motion. The follower magnets are attached to a translating element that presses against a pusher plate that deflects the rubber diaphragm. For further information on this actuator, reference: Massiello et al., "The Cleveland Clinic--Nimbus Total Artificial Heart," Journal of Thoracic and Cardiovascular Surgery, Vol. 108, No. 3, pp. 412-419 (1994); and Harasaki et al., "Progress in Cleveland Clinic--Nimbus Total Artificial Heart Development," ASAIO Journal, M494-M498 (1994).
Although existing energy conversion approaches have been successful to varying extents, the art would be advanced by a next-generation actuator for permanently implantable pulsatile ventricle assist devices and/or total artificial hearts which eliminates mechanical contact and wear between the principal rotary-to-linear motion conversion elements. The present invention provides this advancement.